The present invention relates to a method for heat-treating steel at low cost in order to impart sufficient strength to the steel and also relates to an apparatus for such a heat treatment.
It is known that the types of patenting treatments include lead patenting, molten-salt patenting, fluidized-bed patenting, air patenting, and mist patenting. Off-line patenting treatments mainly employ a lead bed and a fluidized bed. Immediate heat treatments after rolling employ a molten salt, air, and mist.
Lead and a molten salt have a large coefficient of heat transfer, enabling steel to cool rapidly. This is advantageous in obtaining a steel product having high strength. Therefore, they are most effective coolants for obtaining high quality. However, they are not only costly but also produce noxious fumes and toxic substances such as lead oxide because they are used in a lead-bath furnace and a molten-salt furnace. As a result, they are not desirable with respect to the prevention of environmental pollution.
When air and mist are used as a coolant, although they are free from environmental problems, they have a small coefficient of heat transfer and are unable to cool steel rapidly. Therefore, they require to add into the steel material an element that retards the pearlitic transformation in order to obtain a high-strength product. They have another problem in that the product obtained by their use is inferior in strength to a product obtained through lead patenting.
The heat treatment using a fluidized-bed also has a problem of a small coefficient of heat transfer. In the case of wires, this heat-treatment method cannot be applied with sufficient reliability to a wire having a diameter as thick as 2.0 mm or more because sufficient strength may not be obtained.
As described above, in both off-line patenting treatments and immediate heat treatments after rolling, no coolant is known that concurrently satisfies the foregoing three requirements: a large coefficient of heat transfer that enables a steel product to acquire high strength and low cost production, and cause no environmental pollution.
Consequently, the main object of the present invention is to offer a method for heat-treating steel by using a coolant having a large coefficient of heat transfer, at low cost, and which is environment-friendly and to offer an apparatus for such a heat treatment.
The present invention accomplishes the foregoing object by cooling steel in a mixture of solid particles and water.
The mixture may be in a state of suspension in which the solid particles are dispersed in the water. Nevertheless, it is desirable to deposit the solid particles in the water in order to cool the steel in the deposited layer. This method increases the cooling rate, making the cooling more effective.
It is desirable that the solid particles be refractory materials that have high thermal conductivity and that do not deteriorate even in contact with steel at about 900 to 1,000xc2x0 C. Of the refractory materials, oxides are particularly suitable. More specifically, it is desirable to use at least one oxide selected from the group consisting of Al2O3, CaO, MgO, SiO2, ZrO2, ZrO2. SiO2, B2O3, FeO, FeO2, and Fe2O3. In particular, the mixing of iron oxide (FeO, FeO2, or Fe2O3) is effective in preventing the deterioration of the coolant during a prolonged heat treatment. Particles other than the foregoing iron oxide-family sand, such as metal particles and alloy particles, can be used effectively as the solid particles. However, in consideration of a long-term continuous operation, it is desirable to use oxide-family sand considering its resistance to deterioration and corrosion. Graphite powders may also be used as the solid particles. Graphite powders have small specific gravity and high thermal conductivity. Therefore, they are particularly suitable as a coolant for a steel wire disposed on a moving conveyor by a circling laying head for forming a coiling configuration. In the case of substances that tend to coagulate, such as graphite powders, it is desirable to add a surface-active agent to prevent the coagulation.
It is desirable that the solid particles have a specific gravity of 1.0 or more. If the specific gravity is less than 1.0, the solid particles float in the water, making it difficult for the steel to pass through the collection of particles. It is desirable that the specific gravity be 5.0 or less. If more than 5.0, it becomes difficult to insert and carry the steel through the collection of solid particles. In particular, when a heat treatment is carried out for a steel wire disposed on a moving conveyor by a circling laying head for forming a coiling configuration, it becomes difficult to insert and carry the steel wire through the collection of solid particles. It is more desirable that the solid particles have a specific gravity of 3.0 or less. Even a refractory material having a large specific gravity may be used as the solid particles by obtaining a hollow structure in order to reduce the weight per unit volume.
It is desirable that 80 wt % or more of the solid particles have a particle diameter of 1.0 mm or less. If the particle diameter exceeds 1.0 mm, the interstices in which the water can be in direct contact with the steel increases. This increase may cause the nucleate boiling of the water, further enhancing the cooling effect. As a result, an undesirable martensite structure may be formed. In particular, it is desirable that the solid particles have an average particle diameter of 150 xcexcm or less. The average particle diameter of 150 xcexcm or less facilitates the insertion and carrying of the steel even with solid particles having a specific gravity close to 5.0. It is more desirable that the average particle diameter be 100 xcexcm or less.
When the amount of water in the vicinity of the steel is insufficient, the cooling rate of the steel decreases, thereby increasing variations in the strength of the steel in the longitudinal direction. This water deficiency can be prevented by any of the following arrangements:
{circle around (1)} The use of a heat-treatment apparatus comprising (a) a liquid bath that contains water and (b) a solid-particle bath that is partitioned in the liquid bath by a mesh and that contains solid particles. The mesh has openings smaller than the particle diameter of the solid particles. The steel is inserted into the mixture of the solid particles and water in the solid-partide bath so that the steel is cooled.
{circle around (2)} The limitation of the insertion depth of the steel into the mixture of the solid particles and water to 40 cm or less.
{circle around (3)} The forced supply of water between the solid particles in order to prevent the water deficiency between the solid particles in the vicinity of the steel.
In the arrangement {circle around (1)}, since the mesh has openings smaller than the particle diameter of the solid particles, no solid particles escape to the outside of the mesh. Consequently, whereas the solid-particle bath contains the mixture of the solid particles and water, the liquid bath contains water only. There is no specific limitation on the material of the mesh on condition that the mesh can retain the solid particles. It is desirable to use a material such as stainless steel. The dual structure of the heat-treatment apparatus by the use of the mesh enables the solid-particle bath to be surrounded by water at all times, thereby preventing water deficiency in the vicinity of the steel.
It is desirable to stir the water in the liquid bath. The types of the means to stir the water include the rotation of a rotor having a fin in the liquid bath and the formation of a water flow by a pump. The stirring of the water in the liquid bath promotes the penetration of the water into the solid-particle bath, thereby preventing water deficiency in the vicinity of the steel.
In the arrangement {circle around (2)}, it is more desirable that the steel be inserted into the mixture of the solid particles and water (i.e., into the collection of solid particles) at a depth of 25 cm or less, preferably 10 cm or less. The reason is that as the insertion depth increases, it becomes difficult to supply water to the vicinity of the steel in the solid-particle bath.
In the arrangement {circle around (3)}, the formation of a water flow between the solid particles prevents water deficiency in the vicinity of the steel. More specifically, it is desirable that pipe-shaped nozzles be provided in parallel connection at the lower portion of the mixture of the solid particles and water to supply water to the solid particles from the nozzles. Although incapable of fluidizing the solid particles, the supplied water forms a water flow between the solid particles to prevent the water deficiency in the vicinity of the steel.
Additionally, the foregoing water supply may not only form the water flow between the solid particles but also fluidize the solid particles themselves. In order to fluidize the solid particles, mesh-shaped nozzles having numerous small openings may be provided at the lower portion of the mixture of the solid particles and water to supply water from the nozzles. The methods for the fluidization include the supply of water, steam, or air. However, steam and air are not desirable because they form spaces between the solid particles. A stable heat treatment can be conducted only when the fluidization is carried out by the supply of water.
The term xe2x80x9cwaterxe2x80x9d in the coolant includes hot water. It is desirable that the hot water have a temperature of 50xc2x0 C. or higher, more desirably 70xc2x0 C. or higher, preferably 90xc2x0 C. or higher. If lower than 50xc2x0 C., a martensite structure may be formed. The water temperature of 90xc2x0 C. or higher can minimize the variation of the water temperature caused by the temperature variation of the steel, so that the stable heat treatment can be performed.
The heat-treatment method of the present invention may be applied to steel after rolling either on an off-line basis or on an in-line basis in which the steel is immediately heat-treated after the rolling.
The types of the target materials of the heat-treatment method of the present invention include various types of steel. Of these types, carbon steel can be effectively treated. In particular, high-carbon steel is most effectively treated. The heat-treatment method can be applied to any shape including a plate and a wire. In particular, the method is applied to a wire most suitably.
The heat-treatment apparatus of the present invention is an apparatus for heat-treating steel by submerging the steel in a coolant bath. The coolant bath comprises (a) a liquid bath that contains water and (b) a solid-particle bath that is partitioned in the liquid bath by a mesh and that contains solid particles. The mesh has openings smaller than the particle diameter of the solid particles.
It is desirable that the liquid bath be provided with a means for stirring the water. It is also desirable to provide a means for forcibly supplying water between the solid particles. In particular, it is desirable to provide a means for fluidizing the solid particles by the supply of water.